FIG. 13 shows a conventional spark plug 101 which includes a rodlike insulator 11 having an axial hole 10 extending along the direction of an axis G; a center electrode 21 disposed in a front end portion of the axial hole 10; a tubular metallic shell 31 (having a bore extending along the direction of the axis G) which surrounds the circumference of the rodlike insulator 11; and a ground electrode (side electrode) 41 whose one end (proximal end) 42 is joined to the front end 33 of the metallic shell 31 and whose other end (distal end) 43 is bent to face the distal end 23 of the center electrode 21. Notably, reference numeral 35 denotes a screw for attachment to an engine.
Such a spark plug 101 has been manufactured as follows (Japanese Patent Application Laid-Open (kokai) No. 2007-280942). That is, its manufacturing process is as follows. A rod-shaped ground electrode member (a ground electrode intermediate before being bent to form the ground electrode 41), which extends straight along the axis G, is welded at its distal end to the front end 33 of the metallic shell 31. Subsequently, the rodlike insulator holding the center electrode, etc. assembled into the axial hole thereof is assembled into the metallic shell. After that, in order to obtain the spark plug 101 shown in FIG. 13, the ground electrode member is bent such that its distal end 43 faces the distal end 23 of the center electrode 21 with a predetermined spark gap formed therebetween.
Incidentally, in recent years, there has been growing demand for a spark plug whose spark gap has very high dimensional accuracy. Meanwhile, the rodlike insulator 11, fixedly disposed within the metallic shell 31, has a dimensional error of itself and a positional error in the front-rear direction (the direction of the axis G) in relation to the metallic shell 31 stemming from an assembly error. The center electrode 21, fixedly disposed within the rodlike insulator 11, also has a dimensional error of itself and an unavoidable positional error in the front-rear direction stemming from an assembly error. Therefore, the axial distance between the position of the distal end of the unbent ground electrode member, which extends from the front end 33 of the metallic shell 31, and the position of the distal end of the center electrode involves an error even if the length accuracy of the unbent ground electrode member (component) itself is maintained high. That is, even in the case where the ground electrode member which is accurate in length is accurately welded to the front end of the metallic shell, if such a ground electrode member is merely bent, difficulty is encountered in forming a spark gap between the distal end of the ground electrode and the distal end of the center electrode such that the spark gap has a desired high level of dimensional accuracy.
In order to overcome such a problem, the following method has been employed recently (Japanese Patent Application Laid-Open (kokai) No. H08-236263). A ground electrode member (component) before being welded (before being bent) 40 is formed to have a length slightly longer than its designed length, and is welded to the front end 33 of the metallic shell 31 as shown in section A of FIG. 14. The rodlike insulator 11 holding the center electrode 21 is assembled into the metallic shell 31 so as to obtain an intermediate 100 of the spark plug. The amount of projection (in the axial direction) of the unbent ground electrode member 40 in relation to the distal end 23 of the center electrode 21 is rendered slightly greater than the designed amount, and a distal end portion 43b of the ground electrode member 40 is cut before it is bent, whereby the accuracy of the projection amount (dimension) is secured. In this state, the distal end portion 43b of the ground electrode member 40 at the front end 33 of the metallic shell 31 is cut by a slight length (e.g., 0.5 mm to 3 mm) such that the projection dimension of the ground electrode member 40 becomes a desired dimension, whereby the accuracy of the projection dimension is secured. After that, the ground electrode member 40 is bent. Notably, in recent years, demand has arisen for narrowing the ground electrode so as to improve spark performance, and, therefore, there has been performed an operation of cutting and removing opposite sides of the distal end of the ground electrode member 40 such that the distal end has a tapered shape (chamfered shape), rather than merely cutting the distal end portion 43b of the ground electrode member 40 such that the ground electrode member 40 becomes shorter.
Meanwhile, such cutting is effected by means of shearing (work) from the viewpoint of the machining (manufacturing) efficiency. In such shearing work, as shown in section A of FIG. 14, the metallic shell 31 having the ground electrode member 40 welded to its front end 33 is fixed, and a portion of the ground electrode member 40 near the distal end thereof is supported by a stationary blade 50 for cutting, for example, from the outer side (the side opposite the axis G of the metallic shell 31). In this state, as shown in section B of FIG. 14, a cutting blade (movable blade) 60 is fed from the inner side (the side toward the axis G of the metallic shell 31) so as to cut and remove the distal end portion 43b. Notably, such a feed direction of the movable blade 60 is employed so as to cope with the shape of the front end of the spark plug, and to prevent generation of cutting burrs on the spark gap side.